1. Field of the Invention
The present invention relates to a thin film electroluminescence device (TFEL) for use, for example, in an edge emitter of a printer head and, in particular, to a thin film electroluminescence device having an emitting layer of an improved composition.
2. Description of the Reated Art
Generally, a TFEL device comprises a transparent substrate such as a glass plate, one electrode formed on the transparent substrate, an emitting layer sandwiched with two dielectric layers, and the other electrode. The emitting layer is conventionally composed of a host material and a luminescence center as given below:
TABLE 1 ______________________________________ Host Luminescence Center material red green blue bluish green ______________________________________ ZnS Sm.sup.3+ Tb.sup.3+ Tm.sup.3+ CaS Tb.sup.3+ Ce.sup.3+ SrS Sm.sup.3+ Ce.sup.3+ ______________________________________
In the aforementioned TFEL device, when a high electric field of a few MV/cm is applied to the emitting layer upon the application of a voltage between the associated electrodes, electrons are implanted into the emitting layer. At this time, the electrons are accelerated there under the aforementioned high electric field and hot electrons are generated. The hot electrons migrate in the crystal of the host material of the emitting layer, causing them to directly collide with the element ions acting as the luminescence center present in the host material or causing the energy of the hot electrons to be propagated through the crystal lattice of the host material to allow element ions to be excited. As a result, a light emission phenomenon appears.
However, the conventional TFEL device suffers from the following problem. That is, blue light of high colorimetric purity is difficult to produce in the TFEL device having an emitting layer with ZnS as a host material. This is caused by the shifting of an emission spectrum toward a longer wavelength edge because the radius of positive ions (Zn.sup.2+) of the host material ZnS is smaller than that of the lanthanide ions such as Ce.sup.3+ which act as a luminescence center. In the TFEL device having an emitting layer with the alkaline-earth metal's sulfide, such as CaS and SrS, as a host material, it is possible to produce bluish green light since the radius of positive ions (Ca.sup.2+, Sr.sup.2+) in the host material is nearly equal to that of lanthanide element ions such as Ce.sup.3+ which serve as a blue luminescence center. Because of the deliquescence of the alkaline earth metal's sulfide such as CaS and SrS, the emitting layer having such sulphide as the host material reveals a poor stability and an increased aging.
"Phosphor Handbook" Ed, Phosphor Research Society, OHMSHA, LTD, 1978 (p.119) contains a description about the luminescence of CaF.sub.2 ; Sm.sup.2+, SrF.sub.2 ; Sm.sup.2+ and BaF.sub.2 ; Sm.sup.2+. The aforementioned Handbook (pp. 294 to 295) also contains a study on a scintilate phosphor having a peak at 435 nm in a light emission spectrum concerning CaF.sub.2 ; Eu.sup.2+.